1. Field of the Invention
The present invention relates to a process for concurrently fractionating and hydrotreating a full range naphtha stream. More particularly the full boiling range naphtha stream is subjected to simultaneous hydrodesulfurization and splitting into a light boiling range naphtha and a heavy boiling range naphtha and thereafter polishing the light fraction or the recombined light and heavy fraction in a manner to prevent or reduce recombinant mercaptans. The bottoms stream from the CDHDS may also be directed to the countercurrent (side-stripper) polishing reactor to good effect.
2. Related Information
Petroleum distillate streams contain a variety of organic chemical components. Generally the streams are defined by their boiling ranges which determine the compositions. The processing of the streams also affects the composition. For instance, products from either catalytic cracking or thermal cracking processes contain high concentrations of olefinic materials as well as saturated (alkanes) materials and polyunsaturated materials (diolefins). Additionally, these components may be any of the various isomers of the compounds.
The composition of untreated naphtha as it comes from the crude still, or straight run naphtha, is primarily influenced by the crude source. Naphthas from paraffinic crude sources have more saturated straight chain or cyclic compounds. As a general rule most of the xe2x80x9csweetxe2x80x9d (low sulfur) crudes and naphthas are paraffinic. The naphthenic crudes contain more unsaturates and cyclic and polycylic compounds. The higher sulfur content crudes tend to be naphthenic. Treatment of the different straight run naphthas may be slightly different depending upon their composition due to crude source.
Reformed naphtha or reformate generally requires no further treatment except perhaps distillation or solvent extraction for valuable aromatic product removal. Reformed naphthas have essentially no sulfur contaminants due to the severity of their pretreatment for the process and the process itself.
Cracked naphtha as it comes from the catalytic cracker has a relatively high octane number as a result of the olefinic and aromatic compounds contained therein. In some cases this fraction may contribute as much as half of the gasoline in the refinery pool together with a significant portion of the octane. Such cracked-steam sources such as from FCC, coker, visbreaker (and the like) typically contain around 90% of all of the xe2x80x9cdestination sulfurxe2x80x9d that would have reported to refinery gasoline in the absence of all desulfurization treatment.
Catalytically cracked naphtha gasoline boiling range material currently forms a significant part (xcx9c⅓) of the gasoline product pool in the United States and it provides the largest portion of the sulfur. The sulfur impurities may require removal, usually by hydrotreating, in order to comply with product specifications or to ensure compliance with environmental regulations.
The most common method of removal of the sulfur compounds is by hydrodesulfurization (HDS) in which the petroleum distillate is passed over a solid particulate catalyst comprising a hydrogenation metal supported on an alumina base. Additionally copious quantities of hydrogen are included in the feed. The following equations illustrate the reactions in a typical HDS unit:
RSH+H2xe2x86x92RH+H2Sxe2x80x83xe2x80x83(1)
RCl+H2xe2x86x92RH+HClxe2x80x83xe2x80x83(2)
2RN+4H2xe2x86x92RH+NH3xe2x80x83xe2x80x83(3)
ROOH+2H2xe2x86x92RH+H2Oxe2x80x83xe2x80x83(4)
Typical operating conditions for naphtha HDS reactions are:
After the hydrotreating is complete the product may be fractionated or simply flashed to release the hydrogen sulfide and collect the now desulfurized naphtha.
In addition to supplying high octane blending components the cracked naphthas are often used as sources of olefins in other processes such as etherifications. The conditions of hydrotreating of the naphtha fraction to remove sulfur will also saturate some of the olefinic compounds in the fraction, thereby reducing the octane and causing a loss of source olefins.
Various proposals have been made for removing sulfur while retaining the more desirable olefins. Since the olefins in the cracked naphtha are mainly in the low boiling fraction of these naphthas and the sulfur containing impurities tend to be concentrated in the high boiling fraction the most common solution has been prefractionation prior to hydrotreating. The prefractionation produces a light boiling range naphtha which boils in the range of C5 to about 250xc2x0 F. and a heavy boiling range naphtha which boils in the range of from about 250-475xc2x0 F.
The predominant light or lower boiling sulfur compounds are mercaptans while the heavier or higher boiling compounds are thiophenes and other heterocyclic compounds. The separation by fractionation alone will not remove the mercaptans. However, in the past the mercaptans have been removed by oxidative processes involving caustic washing. A combination oxidative removal of the mercaptans followed by fractionation and hydrotreating of the heavier fraction is disclosed in U.S. Pat. No. 5,320,742. In the oxidative removal of the mercaptans the mercaptans are converted to the corresponding disulfides.
In addition to treating the lighter portion of the naphtha to remove the mercaptans it traditionally has been used as feed to a catalytic reforming unit to increase the octane number if necessary. Also the lighter fraction may be subjected to further separation to remove the valuable C5 olefins (amylenes) which are useful in preparing ethers.
U.S. Pat No. 6,083,378 discloses a naphtha splitter as a distillation column reactor to treat a portion or all of the naphtha to remove the organic sulfur compounds contained therein. The catalyst is placed in the distillation column reactor such that the selected portion of the naphtha is contacted with the catalyst and treated. The catalyst may be placed in the rectification section to treat the lighter boiling range components only, in the stripping section to treat the heavier boiling range components only, or throughout the column to widely treat the naphtha. In addition the distillation column reactor may be combined with standard single pass fixed bed reactors or another distillation column reactor to fine tune the treatment.
It has been discovered that during processing, if the H2S is not removed from the catalyst zones quickly problems arise. The H2S can recombine to form mercaptans thus increasing the amount of sulfur in the product. Additional treatment in subsequent units would cause more of the olefins in the feed to be saturated, thus losing octane and consuming hydrogen.
It is an advantage of the present invention that the sulfur may be removed from the light and/or heavy naphtha portions of the stream without any substantial loss of olefins by the recombination of H2S with olefins. Thus, very low levels of sulfur may be obtained in the selected fraction and/or the entire stream. It is a further advantage that most of the H2S dissolved in the light naphtha is removed.
Briefly the present invention is an improvement in a catalytic distillation hydrodesulfurization process comprising:
(a) feeding a naphtha boiling range hydrocarbon stream containing organic sulfur compounds and hydrogen to a distillation column reactor;
(b) concurrently in said distillation column reactor
(i) separating said naphtha into a light boiling range naphtha and a heavy boiling range naphtha
(ii) contacting a fraction of said naphtha and hydrogen with a hydrodesulfurization catalyst to selectively react the organic sulfur compounds therein with said hydrogen to form H2S;
(c) recovering a portion of said light boiling range naphtha wherein said light boiling range naphtha contains recombinant mercaptans;
(d) removing said heavier boiling range naphtha from said distillation column reactor, e.g. as bottoms;
wherein the improvement comprises contacting said portion of said light boiling range naphtha with hydrogen in countercurrent flow in a fixed bed of hydrodesulfurization catalyst to reduce the recombinant mercaptans therein. Another embodiment of the improvement is to direct both the overheads and the bottoms of the catalytic distillation hydrodesulfurization column together through the countercurrent, side-mounted, fixed-bed secondary-hydrodesulfurization polishing reactor.
In the counterflow operation the newly released H2S at a given location is unavailable to react again with olefins in the lower sections of the column to form another mercaptan. Hence, there is substantially no H2S arriving in the bottom of the column and therefore there is no equilibrium limitation on the mercaptan removal.
Since the first column acts as a splitter, either the light naphtha fraction or the heavy naphtha fraction may be hydrodesulfurized in catalytic distillation step. Preferably both fraction are hydrotreated. The H2S produced in the catalytic distillation whether from one or both fractions is removed with the light naphtha fraction, and separated therefrom. Thus, it is in the light naphtha fraction that the recombinant mercaptans are most likely to form, because the H2S will be in contact with that fraction during its recovery.
xe2x80x9cRecombinant mercaptansxe2x80x9d as that term is used herein means those mercaptans which are not in the feed to the present process but are the reaction products of the H2S generated by the hydrogenation of the present process and alkenes in the feed. Thus, the recombinant mercaptans are not necessarily the same as those destroyed by the hydrogenation of the present process, although they may be. The present catalytic distillation hydrogenation is considered to dissociate substantially all of the mercaptans in the feed and the small amounts of mercaptans observed in the product streams are in fact recombinant mercaptans. Although the catalytic distillation reaction is superior to the prior art straight hydrogenation for removing mercaptans, the dynamic system of a catalytic distillation allows sufficient time for some undesirable recombination reaction to occur. Thus, in the present invention the combination of a less efficient countercurrent, straight pass hydrodesulfurization is sufficient to dissociate the small quantities of recombinant mercaptans by having only a limited contact of the produced H2S before it is removed from the reaction zone.
As used herein the term xe2x80x9cdistillation column reactorxe2x80x9d means a distillation column which also contains catalyst such that reaction and distillation are going on concurrently in the column. In a preferred embodiment the catalyst is prepared as a distillation structure and serves as both the catalyst and distillation structure. The term xe2x80x9creactive distillationxe2x80x9d is used to describe the concurrent reaction and fractionation in a column. For the purposes of the present invention, the term xe2x80x9ccatalytic distillationxe2x80x9d includes reactive distillation and any other process of concurrent reaction and fractional distillation in a column regardless of the designation applied thereto.